The solar constant is a measure of the amount of radiant energy reaching the earth's outer atmosphere from the sun. More precisely, it is equal to the rate at which solar radiation falls on a unit area of a plane surface at the top of the atmosphere and oriented at a perpendicular distance of 9.277 x 10⁷ mi (1.496 X 10⁸ km) from the sun. That distance is the average (mean) distance between earth and sun during the course of a single year.

Measuring the solar constant has historically been a difficult challenge since there were few good methods for measuring energy levels at the top of the atmosphere. A solution to this problem was made possible in 1980 with the launching of the Solar Maximum (Solar Max) Mission spacecraft. After repair by the Challenger astronauts in 1984, Solar Max remained in orbit, taking measurements of solar phenomena until 1989. As...